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1.
The Middle Miocene Tsushima granite pluton is composed of leucocratic granites, gray granites and numerous mafic microgranular enclaves (MME). The granites have a metaluminous to slightly peraluminous composition and belong to the calc‐alkaline series, as do many other coeval granites of southwestern Japan, all of which formed in relation to the opening of the Sea of Japan. The Tsushima granites are unique in that they occur in the back‐arc area of the innermost Inner Zone of Southwest Japan, contain numerous miarolitic cavities, and show shallow crystallization (2–6 km deep), based on hornblende geobarometry. The leucocratic granite has higher initial 87Sr/86Sr ratios (0.7065–0.7085) and lower εNd(t) (?7.70 to ?4.35) than the MME of basaltic–dacitic composition (0.7044–0.7061 and ?0.53 to ?5.24), whereas most gray granites have intermediate chemical and Sr–Nd isotopic compositions (0.7061–0.7072 and ?3.75 to ?6.17). Field, petrological, and geochemical data demonstrate that the Tsushima granites formed by the mingling and mixing of mafic and felsic magmas. The Sr–Nd–Pb isotope data strongly suggest that the mafic magma was derived from two mantle components with depleted mantle material and enriched mantle I (EMI) compositions, whereas the felsic magma formed by mixing of upper mantle magma of EMI composition with metabasic rocks in the overlying lower crust. Element data points deviating from the simple mixing line of the two magmas may indicate fractional crystallization of the felsic magma or chemical modification by hydrothermal fluid. The miarolitic cavities and enrichment of alkali elements in the MME suggest rapid cooling of the mingled magma accompanied by elemental transport by hydrothermal fluid. The inferred genesis of this magma–fluid system is as follows: (i) the mafic and felsic magmas were generated in the mantle and lower crust, respectively, by a large heat supply and pressure decrease under back‐arc conditions induced by mantle upwelling and crustal thinning; (ii) they mingled and crystallized rapidly at shallow depths in the upper crust without interaction during the ascent of the magmas from the middle to the upper crust, which (iii) led to fluid generation in the shallow crust. The upper mantle in southwest Japan thus has an EMI‐like composition, which plays an important role in the genesis of igneous rocks there.  相似文献   

2.
The southern Sinai Peninsula, underlain by the northernmost extension of the Arabian-Nubian Shield, exposes post-collisional calc-alkaline and alkaline granites that represent the youngest phase of late Neoproterozoic igneous activity. We report a petrographic, mineralogical and geochemical investigation of post-collisional plutons of alkaline and, in some cases, peralkaline granite. These granites intrude metamorphosed country rocks as well as syn- and post-collisional calc-alkaline granitoids. The alkaline and peralkaline granites of the southern tip of Sinai divide into three subgroups: syenogranite, alkali feldspar granite and riebeckite granite. The rocks of these subgroups essentially consist of alkali feldspar and quartz with variable amounts of plagioclase and mafic minerals. The syenogranite and alkali feldspar granite contain small amounts of calcic amphibole and biotite, often less than 3%, while the riebeckite granite is distinguished by sodic amphibole (5–10%). These plutons have geochemical signatures typical of post-collisional A-type granites and were most likely emplaced during a transition between orogenic and anorogenic settings. The parental mafic magma may be linked to lithospheric delamination and upwelling of asthenospheric mantle material. Differentiation of the underplated basaltic magma with contributions from the juvenile crust eventually yielded the post-collisional alkaline granites. Petrogenetic modelling of the studied granitic suite shows that pure fractional crystallization cannot quantitatively explain chemical variations with the observed suite, with both major oxides and several trace elements displaying trends opposite to those required by the equilibrium phase assemblage. Instead, we show that compositional variation from syenogranite through alkali feldspar granite to riebeckite granite is dominated by mixing between a low-SiO2 liquid as primitive or more primitive than the lowest-SiO2 syenogranite and an evolved, high-SiO2 liquid that might be a high-degree partial melt of lower crust.  相似文献   

3.
The origin of ferroan A-type granites in anorogenic tectonic settings remains a long-standing petrological puzzle. The proposed models range from extreme fractional crystallization of mantle-derived magmas to partial melting of crustal rocks, or involve combination of both. In this study, we apply whole-rock chemical and Sm-Nd isotopic compositions and thermodynamically constrained modeling (Magma Chamber Simulator, MCS) to decipher the genesis of a suite of A1-type peralkaline to peraluminous granites and associated intermediate rocks (monzodiorite-monzonite, syenite) from the southwestern margin of the Archean Karelia craton, central Finland, Fennoscandian Shield. These plutonic rocks were emplaced at ca. 2.05 Ga during an early stage of the break-up of the Karelia craton along its western margin and show trace element affinities to ocean island basalt-type magmas. The intermediate rocks show positive εNd(2050 Ma) values (+1.3 to +2.6), which are only slightly lower than the estimated contemporaneous depleted mantle value (+3.4), but much higher than average εNd(2050 Ma) of Archean TTGs (–10) in the surrounding bedrock, indicating that these rocks were essentially derived from a mantle source. The εNd(2050 Ma) values of the peralkaline and peraluminous granite samples overlap (–0.9 to +0.6 and –3.2 to +0.9, respectively) and are somewhat lower than those in the intermediate rocks, suggesting that the mafic magmas parental to granite must have assimilated some amount of older Archean continental crust during their fractionation, which is consistent with the continental crust-like trace element signatures of the granite members. The MCS modeling indicates that fractional crystallization of mantle-derived magmas can explain the major element characteristics of the intermediate rocks. The generation of the granites requires further fractional crystallization of these magmas coupled with assimilation of Archean crust. These processes took place in the middle to upper crust (∼2–4 kbar, ∼7–15 km) and involved crystallization of large amounts of clinopyroxene, plagioclase and olivine. Our results highlight the importance of coupled FC-AFC processes in the petrogenesis of A-type magmas and support the general perception that magmas of A-type ferroan granites become more peraluminous by assimilation of crust. They further suggest that variable fractionation paths of the magmas upon the onset of assimilation may explain the broad variety of A-type felsic and intermediate igneous rocks that is often observed emplaced closely in time and space within the same igneous complex.  相似文献   

4.
Two Late Neoproterozoic post-collisional igneous suites, calc-alkaline (CA) and alkaline–peralkaline (Alk), widely occur in the northernmost part of the Arabian–Nubian Shield. In Sinai (Egypt) and southern Israel they occupy up to 80% of the exposed basement. Recently published U–Pb zircon geochronology indicates a prolonged and partially overlapping CA and Alk magmatism at 635–590 Ma and 608–580 Ma, respectively. Nevertheless in each particular locality CA granitoids always preceded Alk plutons. CA and Alk igneous rocks have distinct chemical compositions, but felsic and mafic rocks in general and granitoids from the two suites in particular cannot be distinguished by their Nd, Sr and O isotope ratios. Both suites are characterized by positive εNd(T) values, from + 1.5 to + 6.0 (150 samples, 28 of them are new analyses), but predominance of juvenile crust in the region prevents unambiguous petrogenetic interpretation of the isotope data. Comparison of geochemical traits of felsic and mafic rocks in each suite suggests a significant contribution of mantle-derived components to the silicic magmas. Model calculation shows that the alkaline granite magma could have been produced by partial (~ 20%) melting of rocks corresponding to K-rich basalts. Material balance further suggests that granodiorite and quartz monzonite magmas of the CA suite could form by mixing of the granite and gabbro end-members at proportions of 85/15. In the Alk suite, alkali feldspar and peralkaline granites have evolved mainly by fractional crystallization of feldspars and a small amount of mafic minerals from a parental syenogranite melt. Thus the protracted, 20 m.y. long, contemporaneous CA and Alk magmatism in the northern ANS requires concurrent tapping of two distinct mantle sources. Coeval emplacement of CA and Alk intrusive suites was described in a number of regions throughout the world.  相似文献   

5.
拉萨地块北部阿索地区新发现的晚白垩世晚期基性岩墙对于讨论班公湖-怒江洋盆闭合后的碰撞过程具有重要研究意义。本文报道了阿索地区目思基性岩墙的全岩地球化学组成和LA-ICP-MS锆石U-Pb年龄。研究表明,目思基性岩墙中一组最年轻锆石的U-Pb加权平均年龄为74Ma。岩墙内的辉绿玢岩SiO2含量介于51.90%~53.55%之间,MgO含量介于3.98%~4.97%之间,Mg^#为50.0~57.5,低Cr(51.30×10^-6~79.48×10^-6)和Ni(55.94×10^-6~74.17×10^-6)含量。岩墙具有轻稀土元素富集的特征,明显的负Eu异常。在微量元素方面,富集Ba、Th、U、K等大离子亲石元素富集,亏损Nb、Ta、Ti等高场强元素,并具有Pb的正异常。La/Sm-Sm/Yb投图结果显示基性岩墙来自于尖晶石+石榴石二辉橄榄岩地幔源区1%~5%的部分熔融,并在演化过程中发生了橄榄石、单斜辉石及斜长石的结晶分异,同时在岩浆上升过程中受到地壳混染。结合区域地质背景,拉萨地块与羌塘地块碰撞后下地壳增厚并发生榴辉岩化,导致拆沉作用。目思基性岩墙的锆石U-Pb年龄表明拆沉作用持续到晚白垩世晚期。  相似文献   

6.
Mafic and intermediate intrusions occur in the Slavkovsky les as dykes, sills and minor tabular bodies emplaced in metamorphic rocks or enclosed in late Variscan granites near the SW contact of the Western Krušné hory/Erzgebirge granite pluton. They are similar in composition and textures to the redwitzites defined in NE Bavaria. Single zircon Pb-evaporation analyses constrain the age of a quartz monzodiorite at 323.4 ± 4.4 Ma and of a granodiorite at 326.1 ± 5.6 Ma. The P–T range of magma crystallization is estimated at ~1.4–2.2 kbar and ~730–870°C and it accords with a shallow intrusion level of late Variscan granites but provides lower crystallization temperatures compared to the Bavarian redwitzites. We explain the heterogeneous composition of dioritic intrusions in the Slavkovsky les by mixing between mafic and felsic magmas with a minor effect of fractional crystallization. Increased K, Ba, Rb, Sr and REE contents compared to tholeiitic basalts suggest that the parental mafic magma was probably produced by melting of a metasomatised mantle, the melts being close to lamprophyre or alkali basalt composition. Diorites and granodiorites originated from mixed magmas derived by addition of about 25–35 and 50 vol.%, respectively, of the acid end-member (granite) to lamprophyre or alkali-basalt magma. Our data stress an important role of mafic magmas in the origin of late Variscan granitoids in NW Bohemian Massif and emphasize the effect of mantle metasomatism on the origin of K-rich mafic igneous rocks.  相似文献   

7.
We present field and petrographic data on Mafic Magmatic Enclaves (MME), hybrid enclaves and synplutonic mafic dykes in the calc-alkaline granitoid plutons from the Dharwar craton to characterize coeval felsic and mafic magmas including interaction of mafic and felsic magmas. The composite host granitoids comprise of voluminous juvenile intrusive facies and minor anatectic facies. MME, hybrid enclaves and synplutonic mafic dykes are common but more abundant along the marginal zone of individual plutons. Circular to ellipsoidal MME are fine to medium grained with occasional chilled margins and frequently contain small alkali feldspar xenocrysts incorporated from host. Hybrid magmatic enclaves are intermediate in composition showing sharp to diffused contacts with adjoining host. Spectacular synplutonic mafic dykes commonly occur as fragmented dykes with necking and back veining. Similar magmatic textures of mafic rocks and their felsic host together with cuspate contacts, magmatic flow structures, mixing, mingling and hybridization suggest their coeval nature. Petrographic evidences such as disequilibrium assemblages, resorption, quartz ocelli, rapakivi-like texture and poikilitically enclosed alkali feldspar in amphibole and plagioclase suggest interaction, mixing/mingling of mafic and felsic magmas. Combined field and petrographic evidences reveal convection and divergent flow in the host magma chamber following the introduction of mafic magmas. Mixing occurs when mafic magma is introduced into host felsic magma before initiation of crystallization leading to formation of hybrid magma under the influence of convection. On the other hand when mafic magmas inject into host magma containing 30–40% crystals, the viscosities of the two magmas are sufficiently different to permit mixing but permit only mingling. Finally, if the mafic magmas are injected when felsic host was largely crystallized (~70% or more crystals), they fill early fractures and interact with the last residual liquids locally resulting in fragmented dykes. The latent heat associated with these mafic injections probably cause reversal of crystallization of adjoining host in magma chamber resulting in back veining in synplutonic mafic dykes. Our field data suggest that substantial volume of mafic magmas were injected into host magma chamber during different stages of crystallization. The origin of mafic magmas may be attributed to decompression melting of mantle associated with development of mantle scale fractures as a consequence of crystallization of voluminous felsic magmas in magma chambers at deep crustal levels.  相似文献   

8.
Late Neoproterozoic bimodal dyke suites are abundant in the Arabian–Nubian Shield. In southern Israel this suite includes dominant alkaline quartz porphyry dykes, rare mafic dykes, and numerous composite dykes with felsic interiors and mafic margins. The quartz porphyry chemically corresponds to A-type granite. Composite dykes with either abrupt or gradational contacts between the felsic and mafic rocks bear field, petrographic and chemical evidence for coexistence and mixing of basaltic and rhyolitic magmas. Mixing and formation of hybrid intermediate magmas commenced at depth and continued during emplacement of the dykes. Oxygen isotope ratios of alkali feldspar in quartz porphyry (13 to 15‰) and of plagioclase in trachydolerite (10–11‰) are much higher than their initial magmatic ratios predicted by equilibrium with unaltered quartz (8 to 9‰) and clinopyroxene (5.8‰). The elevation of δ18O in alkali feldspar and plagioclase, and extensive turbidization and sericitization call for post-magmatic low-temperature (≤ 100 °C) water–rock interaction. Hydrous alteration of alkali feldspar, the major carrier of Rb and Sr in the quartz–porphyry, also accounts for the highly variable and unusually high I(Sr) of 0.71253 to 0.73648.

The initial 143Nd/144Nd ratios, expressed by εNd(T) values, are probably unaltered and show small variation in mafic and felsic rocks within a narrow range from + 1.4 to + 3.3. The Nd isotope signature suggests either a common mantle source for the mafic and silicic magmas or a juvenile crustal source for the felsic rocks (metamorphic rocks from the Elat area). However, oxygen isotope ratios of zircon in quartz porphyry [δ18O(Zrn) = 6.5 to 7.2‰] reveal significant crustal contribution to the rhyolite magma, suggesting that mafic and A-type silicic magmas are not co-genetic, although coeval. Comparison of 18O/16O ratios in zircon allows to distinguish two groups of A-type granites in the region: those with mantle-derived source, δ18O(Zrn) ranging from 5.5 to 5.8‰ (Timna and Katharina granitoids) and those with major contribution of the modified juvenile crustal component, δ18O(Zrn) varying from 6.5 to 7.2‰ (Elat quartz porphyry dykes and the Yehoshafat alkaline granite). This suggests that A-type silicic magmas in the northern ANS originated by alternative processes almost coevally.  相似文献   


9.
South Indian granulite terrain had witnessed significant part of Precambrian mafic igneous activity in the form of episodic mafic dyke intrusions of the Palaeoproterozoic period. Strike trends of these dykes are not uniform over the region and the dykes are generally fresh, massive, black dolerites except in the Bhavani shear zone bordering the southern fringes of Nilgiri massif. In Agali-Coimbatore area of our study in the western Bhavani shear zone, the dykes appear to be penecontemporaneous with shearing. Isotopic data place age of Agali-Coimbatore dyke intrusions at about 2.1 Ga. The age of these dykes is significant to constrain an early Palaeoproterozoic age for major shearing event in the Bhavani shear zone. Other dyke emplacement ages are placed at about 1.8 Ga and 1.65 Ga based on the Ar/Ar and K-Ar isotopic results of dykes in Dharmapuri and Tiruvannamalai areas. Older ages comparable to those of the Dharwar craton are not known and in this respect future isotopic dating is vital. Geochemically, these dykes are quartz/hypersthene normative subalkalic tholeiites. An attempt is made here to provide insights into the general petrogenetic history of the Precambrian dykes. Compositional trends are explained by the fractional crystallization of ferromagnesian phases and plagioclase control is conspicuous at the advanced stages of fractionation. Geochemical characteristics suggest that the dykes have tapped Fe-rich non-pyrolite mantle sources with LIL and LREE enrichment as in many continental basalts. The data suggest that role of crustal contamination is limited in petrogenesis; crustal signatures are noticed in the more mafic end members formed in early stage of evolution suggesting that contamination was temperature controlled with most primitive high temperature magmas being most vulnerable to the process. Nd-Sr isotopic data, at present restricted to Agali-Coimbatore dykes, suggest that Palaeoproterozoic magmas tapped subcontinental lithosphere that may have stabilized in the Archaean times at about 3 Ga during the major crustal building activity in the shield region. Further work coupled with isotopic and mineral chemistry will improve our knowledge on the petrological evolution of the dyke magmas and mafic magmatism in general.  相似文献   

10.
G.F. Zellmer  S.P. Turner 《Lithos》2007,95(3-4):346-362
Mafic enclaves are commonly found in intermediate arc magmas, and their occurrence has been linked to eruption triggering by pre-eruptive magma mixing processes. New major, trace, Sr–Nd and U–Th isotope data of rocks from Nisyros in the Aegean volcanic arc are presented here. Pre-caldera samples display major and trace element trends that are consistent with fractionation of magnetite and apatite within intermediate compositions, and zircon within felsic compositions, and preclude extensive hybridization between mafic and felsic magmas. In contrast, post-caldera dacites form a mixing trend towards their mafic enclaves. In terms of U-series isotopes, most samples show small 238U excesses of up to  10%. Mafic enclaves have significantly higher U/Th ratios than their dacitic host lavas, precluding simple models that relate the mafic and felsic magmas by fractionation or aging alone. A more complicated petrogenetic scenario is required. The post-caldera dacites are interpreted to represent material remobilized from a young igneous protolith following influx of fresh mafic magma, consistent with the U–Th data and with Sr–Nd isotope constraints that point to very limited (< 10%) assimilation of old crust at Nisyros. When these results are compared to data from Santorini in the same arc, there are many geochemical similarities between the two volcanic centers during the petrogenesis of the pre-caldera samples. However, striking differences are apparent for the post-caldera lavas: in Nisyros, dacites show geochemical and textural evidence for magma mixing and remobilization by influx of mafic melts, and they erupt as viscous lava domes; in Santorini, evidence for geochemical hybridization of dacites and mafic enclaves is weak, dacite petrogenesis does not involve protolith remobilization, and lavas erupt as less viscous flows. Despite these differences, it appears that mafic enclaves in intermediate Aegean arc magmas consistently yield timescales of at least 100 kyrs between U enrichment of the mantle wedge and eruption, on the upper end of those estimated for the eruptive products of mafic arc volcanoes. Finally, the data presented here provide constraints on the rates of differentiation from primitive arc basalts to dacites (less than  140 kyrs), and on the crustal residence time of evolved igneous protoliths prior to their remobilization by mafic arc magmas (greater than  350 kyrs).  相似文献   

11.
最近,花岗岩混合成了花岗岩研究的热点,国内外许多学者探讨了花岗岩混合问题,并尝试用不同端元组分不同比例的混合来解释花岗岩的地球化学变化。本文从花岗岩与玄武岩的对比出发,探讨了花岗岩混合的可能性和局限性。作者认为,花岗岩混合的现象是普遍存在的,但是次要的和局部的。岩浆混合的能力或能干性(competence of mixing)主要取决于岩浆的黏性和温度,而黏性又与硅氧四面体有关。相对于玄武岩,花岗岩的SiO_2含量高,温度低,因此,花岗质岩浆的混合能干性很低。玄武质岩浆的混合是mixing(以化学混合为主),而花岗质岩浆的混合通常只是mingling(以机械混合为主),只有在少数情况下才能达到mixing的程度,例如,埃达克岩与地幔混合形成的高镁安山岩或高镁埃达克岩。许多人认为,花岗岩中的暗色微粒包体是花岗质岩浆混合作用最显著、最直接证据。研究表明,花岗岩中的暗色微粒包体大多是闪长质成分的,其初始成分大多是玄武质的。因此,暗色微粒包体不是花岗质岩浆混合作用最显著、最直接证据,而是玄武质岩浆混合能力强过花岗质岩浆的证据。与玄武质岩浆的起源比较,花岗质岩浆从一开始熔融就是不均一的,这源于源区的不均一及熔融过程的复杂性。花岗质岩浆原始均一性的假定是不可能的。花岗岩成分的变化以及在哈克图解中成分点的"连续谱系",主要是由源区不均一性引起的,混合和分异可能有一定的作用,但毕竟是次要的。花岗质岩浆从源区生成、迁移、直至在地表喷出或在浅部定位的全过程,是一个不断均一化和不均一化的过程。但是,由于花岗质岩浆的黏性大,上述过程及岩浆演化的程度和规模都受到限制,也限制了岩浆混合的程度和规模。许多人仅从花岗岩地球化学成分的变化来研究花岗岩的成因,而很少考虑花岗岩物理性质对岩浆演化的制约。对比玄武岩与花岗岩,我们认为,地球化学方法在花岗岩中应用的范围和程度可能远远不及玄武岩,我们应当重新考虑花岗岩的地球化学应用问题。  相似文献   

12.
G. Capaldi  S. Chiesa  P. Manetti  G. Orsi  G. Poli 《Lithos》1987,20(6):433-444
North Yemen (Y.A.R.) was affected, from 30 to 20 Ma ago, by intense magmatic activity (Trap Series), contemporary with the main tensional tectonic phases of the Afro-Arabian Plate. In the Trap activity, two main phases have been recognized; the former, mainly basaltic, between 30 and 26 Ma; and the second, mainly acidic, between 23 and 20 Ma. During the latter period of activity, a basaltic dyke swarm and stock-like bodies and dykes (granophyres) were emplaced along the escarpment and coastal plain. Some granitic bodies intruded the Trap Series towards the end of the first phase and during the second one.

On the basis of geochemical and petrological data, the Tertiary granites of Yemen belong to alkaline or peralkaline associations of A-type. The textural characteristics indicate that the granites are either hypersolvus or subsolvus. The mafic mineral assemblages allow three main types of granites to be distinguished by their different environments of crystallization.

As regards the origin of these kinds of granites, several hypotheses account for the peculiarities observed. The lack of crustal xenoliths, the preliminary data on the Sr isotopic ratio, the data on major and trace elements point to a direct origin by fractional crystallization, without substantial crustal interaction. This agrees well with the strong tensile tectonics, linked with the uplift of the Afro-Arabian dome and with the lithosphere thinning, which provide a minimal opportunity for interaction of the magmas with the crust.  相似文献   


13.
The Nagar Parkar Igneous Complex consists of Neoproterozoic igneous and metamorphic rocks dissected by mafic, felsic, and rhyolitic dykes. The latter can be classified broadly into porphyritic felsic dykes intruding gray and pink granites at Nagar Parkar and the surrounding areas, and the orthophyric felsic dykes intruding amphibolites, deformed pink granites, and the alkaline mafic dykes in the Dhedvero area, north of Nagar Parkar. The porphyritic felsic dykes are composed of perthites, quartz, and albitic plagioclase whereas the orthopheric felsic dykes contain K-feldspar (dominant), plagioclase, and minor quartz. Geochemically, the porphyritic and orthophyric felsic dykes are subalkaline and alkaline demonstrating post-orogenic A2- and OIB-A1-type characteristic on Nb–Y–Ce and Nb–Y–3Ga ternary plots, respectively. One orthophyric felsic dyke contains normative acmite and sodium metasilicate. This study suggests two distinct tectonic regimes for the origin of the felsic dykes of the area. The porphyritic felsic dykes show similarities with the ~800–700 Ma granites of the area, the rhyolite dykes of the Mount Abu, western Rajasthan in India, and the granites of the Seychelles microcontinent. The orthophyric felsic dykes show chemical resemblance with the Tavidar volcanic suite of western Rajasthan and the Silhouette and North islands of the Seychelles microcontinent. This study confirms spatial and temporal links among the Rodinian fragments exposed in the Nagar Parkar area of Pakistan, western Rajasthan of India, and the Seychelles microcontinent.  相似文献   

14.
The Tin Zebane dyke swarm was emplaced at the end of the Pan-African orogeny along a mega-shear zone separating two contrasting terranes of the Tuareg shield. It is located along the western boundary of the Archaean In Ouzzal rigid terrane, but inside the adjacent Tassendjanet terrane, strongly remobilized at the end of the Precambrian. The Tin Zebane swarm was emplaced during post-collisional sinistral movements along the shear zone at 592.2±5.8 Ma (19WR Rb–Sr isochron). It is a dyke-on-dyke system consisting of dykes and stocks of gabbros and dykes of metaluminous and peralkaline granites. All rock types have Sr and Nd isotopic initial ratios (Sri=0.7028 and Nd=+6.2) typical of a depleted mantle source, similar to the prevalent mantle (PREMA) at that period. No crustal contamination occurred in the genesis of the Tin Zebane swarm. Even the samples showing evidence of fluid interaction (essentially alkali mobility) have the same isotopic signature. The peralkaline granites have peculiar geochemical characteristics that mimic subduction-related granites: this geochemical signature is interpreted in terms of extensive differentiation effects due to late cumulates comprising aegirine, zircon, titanite, allanite and possibly fergusonite, separated from the liquid in the swarm itself due to magmatic flow turbulence. The Tin Zebane dyke swarm is thus of paramount importance for constraining the differentiation of mantle products to generate highly evolved alkaline granites without continental crust participation, in a post-collisional setting.  相似文献   

15.
Field and petrographic studies are carried out to characterize the interactions of mafic and felsic magmas from Pithora region of the northeastern part of the Bastar Craton. The MMEs, syn-plutonic mafic dykes, cuspate contacts, magmatic flow textures, mingling and hybridization suggest the coeval emplacement of end member magmas. Petrographic evidences such as disequilibrium assemblages, resorption textures, quartz ocelli, rapakivi and poikilitic textures suggest magma mingling and mixing phenomena. Such features of mingling and mixing of the felsic and mafic magma manifest the magma chamber processes. Introduction of mafic magmas into the felsic magmas before initiation of crystallization of the latter, results in hybrid magmas under the influence of thermal and chemical exchange. The mechanical exchange occurs between the coexisting magmas due to viscosity contrast, if the mafic magma enters slightly later into the magma chamber, then the felsic magma starts to crystallize. Blobs of mafic magma form as MMEs in the felsic magma and they scatter throughout the pluton due to convection. At a later stage, if mafic magma enters the system after partial crystallization of felsic phase, mechanical interaction between the magmas leads to the formation of fragmented dyke or syn-plutonic mafic dyke. All these features are well-documented in the study area. Field and petrographic evidences suggest that the textural variations from Pithora region of Bastar Craton are the outcome of magma mingling, mixing and hybridization processes.  相似文献   

16.
The relationship between plate tectonics and the reworking of continental crust remains controversial. Multistage, hornblende-free, S-type granites across the Malay Peninsula Sn belt are ideal for investigating this research question. Here we present zircon U-Pb ages, in-situ apatite Nd and zircon Hf isotope data, and whole-rock major and trace element data for these S-type granites and spatially associated dykes. Four generations of Permian–Triassic (276–272, 262–260, 231–222, and 202 Ma) S-type granites were identified. The different S-type granites show distinct in-situ zircon Hf and apatite Nd isotopic compositions, implying generation from different sedimentary protoliths. Input of mantle-derived components for the formation of all these S-type granites, further indicating that both continental crustal reworking and growth occurred in the Malay Peninsula during the Permian–Triassic. A 250 Ma dolerite dyke in the Eastern Province was derived from an E-MORB-like mantle source. However, a 202 Ma monzonite dyke in the Western Province, was derived from mafic magmas produced by the melting of enriched mantle, followed by subsequent incorporation of crustal materials. All these S-type granitic magmas were reduced that inherited from sedimentary protoliths, which were favourable for Sn mineralization. Apatite F-Cl concentrations and F/Cl ratios in the S-type granites and related dykes changed systematically through time. We infer that the formation of these S-type granites and related dykes corresponds to the Palaeo-Tethyan evolution (i.e., early subduction of Palaeo-Tethyan oceanic lithosphere and subsequent collision between the Sibumasu and Indochina blocks). Our study also support that multistage S-type granites can be generated in distinct tectonic environments at different times in the same region.  相似文献   

17.
We present a first overview of the synplutonic mafic dykes (mafic injections) from the 2.56–2.52 Ga calcalkaline to potassic plutons in the Eastern Dharwar Craton (EDC). The host plutons comprise voluminous intrusive facies (dark grey clinopyroxene-amphibole rich monzodiorite and quartz monzonite, pinkish grey porphyritic monzogranite and grey granodiorite) located in the central part of individual pluton, whilst subordinate anatectic facies (light grey and pink granite) confined to the periphery. The enclaves found in the plutons include highly angular screens of xenoliths of the basement, rounded to pillowed mafic magmatic enclaves (MME) and most spectacular synplutonic mafic dykes. The similar textures of MME and adjoining synplutonic mafic dykes together with their spatial association and occasional transition of MME to dismembered synplutonic mafic dykes imply a genetic link between them. The synplutonic dykes occur in varying dimension ranging from a few centimeter width upto 200 meters width and are generally dismembered or disrupted and rarely continuous. Necking of dyke along its length and back veining of more leucocratic variant of the host is common feature. They show lobate as well as sharp contacts with chilled margins suggesting their injection during different stages of crystallization of host plutons in magma chamber. Local interaction, mixing and mingling processes are documented in all the studied crustal corridors in the EDC. The observed mixing, mingling, partial hybridization, MME and emplacement of synplutonic mafic dykes can be explained by four stage processes: (1) Mafic magma injected during very early stage of crystallization of host felsic magma, mixing of mafic and felsic host magma results in hybridization with occasional MME; (2) Mafic magma introduced slightly later, the viscosities of two magmas may be different and permit only mingling where by each component retain their identity; (3) When mafic magma injected into crystallizing granitic host magma with significant crystal content, the mafic magma is channeled into early fractures and form dismembered synplutonic mafic dykes and (4) Mafic injections enter into largely crystallized (>80% crystals) granitic host results in continuous dykes with sharp contacts. The origin of mafic magmas may be related to development of fractures to mantle depth during crystallization of host magmas which results in the decompression melting of mantle source. The resultant hot mafic melts with low viscosity rise rapidly into the crystallizing host magma chamber where they interact depending upon the crystallinity and viscosity of the host. These hot mafic injections locally cause reversal of crystallization of the felsic host and induce melting and resultant melts in turn penetrate the crystallizing mafic body as back veining. Field chronology indicates injection of mafic magmas is synchronous with emplacement of anatectic melts and slightly predates the 2.5 Ga metamorphic event which affected the whole Archaean crust. The injection of mafic magmas into the crystallizing host plutons forms the terminal Archaean magmatic event and spatially associated with reworking and cratonization of Archaean crust in the EDC.  相似文献   

18.
近代花岗岩研究的回顾   总被引：19，自引：1，他引：18  
近代花岗岩实验和地球化学的研究使花岗岩研究走向一个最终成因和在地构造环境变迁结合的动力学方向。  相似文献   

19.
王超  刘良  张安达  杨文强  曹玉亭 《岩石学报》2008,24(12):2809-2819
阿尔金造山带南缘玉苏普阿勒克塔格岩体中的似斑状中粗粒黑云钾长花岗岩发育有岩浆成因的暗色包体,并且该花岗岩被花岗细晶岩呈脉状侵入。该岩体含有丰富的岩浆混合作用特征: 如暗色包体中的碱性长石斑晶、针状磷灰石、长石的环斑结构、石英/斜长石主晶和榍石眼斑等。暗色包体、寄主花岗岩和花岗细晶岩代表了岩浆混合演化过程中不同端元比例混合的产物。地球化学特征上,钾长花岗岩和暗色包体的主要氧化物含量在Harker图解中多呈线性变化。暗色包体主要为闪长质,MgO、K2O含量高,为钾玄岩系列,总体上高场强元素不亏损,显示了岩浆混合中的基性端元信息,可能为幔源熔体结晶分异或壳幔物质的混合产物。寄主花岗岩均为准铝质,富碱,为高钾钙碱性系列,亏损Nb、Ta、Sr、P、Ti等高场强元素,高K2O/Na2O,富集高不相容元素,Ga含量高,显示了A型花岗岩的特征,Th/U 和Nb/Ta比值分别介于为6.67~10.96、8.99~11.94,代表了下地壳源区。花岗细晶岩均为钠质、过铝质,TiO2、MgO含量低, Na2O和CaO含量高,具有混合岩浆侵位后分异的特征。岩相学和地球化学特征说明岩浆混合作用对于环斑结构花岗岩的形成起到重要作用。花岗细晶岩中环斑长石的斜长石外环与钾长石内核的厚度比大于钾长花岗岩中的环斑长石,指示混合岩浆在一定的减压条件下更有利于环斑结构的形成。玉苏普阿勒克塔格岩体中的钾玄质暗色包体、高钾钙碱性花岗岩和中钾钙碱性花岗细晶岩代表了岩浆演化不同阶段的产物,反映了一个幔源岩浆和下地壳不断相互作用,引起地壳连续伸展减薄的过程,指示阿尔金南缘在早古生代末期存在造山后伸展背景下的幔源岩浆底侵作用。同一岩体中两种不同时代岩性的环斑结构显示了该岩体形成历史中的一定时空演化关系,代表了伸展过程中不同阶段的产物。  相似文献   

20.
Kharsar hill is one of many granitic plutons comprising the Nagar Parkar igneous complex. The eastern part of the hill is occupied by grey-pink granite (earlier) and the western part by pink granite (later). They are composed of perthite, quartz, and plagioclase, with minor opaque oxide, biotite, titanite, local amphibole, and secondary chlorite, epidote, leucoxene/titanite. The pink granite is characterized by the presence of mafic clots. Both the granitoids are intruded by microgranite/aplite, and porphyritic mafic and rhyolite dykes, locally in swarms. These are abundant in a NE trending 200 m wide zone cutting the entire granite hill. The dykes may extend over 1 km in length and >10 m in thickness, but most are < 100 m in length. The felsic dykes are of several generations; some are associated with the two varieties of granite, others are contemporaneous with the rhyolite and mafic dykes. The mafic dykes can be grouped into two types one of which contains hornblende and the other augite as the principal mafic mineral. Major element analyses suggest that the granitic rocks are metaluminous. The Kharsar granites, like the others in Nagar Parkar, may be an extension of the Malani igneous suite of Rajasthan. The occurrence of bimodal mafic-felsic dykes and petrographic variation in the mafic dykes are briefly discussed.  相似文献   

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